1. Field of the Invention
The present invention relates to methods and devices for imaging in digital dental radioscopy, and especially to methods and devices making use of sensor arrays whose image elements are smaller than would be necessary for the desired local resolution.
2. Description of Prior Art
Dental X-ray diagnostic units in which images are produced digitally making use of CCD sensors (CCD =charge coupled device) have been known for some time. The sensors used normally comprise the CCD sensor itself and a scintillation layer which is applied directly to the CCD sensor and which converts the incident X-radiation into visible light which will then generate an electric charge in the underlying CCD sensor. The image information is then read from the CCD sensor by applying suitable clock signals, whereupon it is preprocessed, digitized and finally transmitted to a computer system, e.g. a personal computer, for display and storage. In comparison with a conventional film technique, this method is primarily advantageous insofar as images are obtained much faster.
An example of such X-ray diagnostic units is described in DE-A-19615178. The diagnostic unit described in this publication additionally comprises a correction means for compensating fluctuations of the electric signals of the individual elements of the CCD array caused by a dark current of these elements, by different conversion efficiencies of these elements and by inhomogeneities of the scintillation layer.
The size of the image elements of the CCD sensor, which are also referred to as pixels, is normally adapted to the desired local resolution. Furthermore, image elements which are smaller than the desired local resolution can be used, image information of neighbouring image elements being then already combined on the sensor. This course of action is referred to as binning. An effective image element whose size corresponds again to the desired local resolution is obtained in this way. The resolution demanded in the field of digital dental radioscopy is typically a resolution of from 50 xcexcm to 60 xcexcm. A higher resolution will normally not make sense, since the scintillator required for converting the X-radiation into visible light will normally not permit a higher resolution. It is true that a higher resolution can be achieved by particularly thin scintillation layers, but this is disadvantageous insofar as only a very small percentage of the X-radiation can be detected by means of these very thin scintillation layers and, due to a correspondingly lower signal-to-noise ratio, this will lead to a deterioration of the image quality.
The necessary total area of such a sensor for dental radioscopy amounts to several square centimetres, since, if the total area of the sensor is smaller than that, it will be difficult to show a whole tooth on a single X-ray image. The size of the sensors is normally adapted to the size of conventional tooth films, which usually starts with a size of 2xc3x973 cm2.
Known dental radioscopy techniques have a plurality of disadvantages. On the one hand, the image quality of digital dental X-ray photographs is usually not yet satisfactory, since the image is very noisy. One reason for this is that only a small number of X-ray quanta per image element contributes to the production of the image, since the patient should only be exposed to a small dose for medical reasons; this leads to quantum noise. In addition to this quantum noise, a second source of noise exists, since only part of the X-radiation falling on the X-ray sensor is absorbed by the scintillation layer and converted into visible light, whereas part of the X-radiation passes through the scintillation layer and can be absorbed in the CCD sensor itself, the CCD sensor consisting preferably of silicon. Such an absorption of X-ray quanta in the silicon of the CCD sensor will be referred to as xe2x80x9cdirect hitxe2x80x9d in the following. This effect is undesired, since X-ray quanta converted in the scintillation layer generate only a small charge in the CCD sensor, typically a charge in the range of a few hundred electrons. Direct hits, however, generate in comparison therewith a very high charge, typically a charge in the range of ten thousands of electrons. Hence, a direct hit will typically produce a charge which is 50 times as high as an X-ray quantum converted in the scintillator. Direct hits will therefore contribute to image noise to a very high extent.
Another problem of known sensors is to be seen in that, only in the most advantageous cases will it be possible to produce sensors of the above-mentioned geometrical size which do not have any fault whatsoever. Frequently, individual image elements or several neighbouring image elements, so-called clusters, or even complete lines or columns of the sensor will fail to contribute. In this connection defects occur in the case of which the defective image element or pixel generates a large image signal without exposure as well as defects in the case of which the defective image element, though exposed, does not generate any image signal. Depending on the number and kind of such defects, a distinction is made between various sensor qualities which are suitable for different cases of use. When the number of defects is excessively large, i.e. when the quality of the sensor is too poor, the sensor cannot be employed for the intended use, i.e. for dental radioscopy, since, due to the defective image elements, the dentist may perhaps fail to notice important image information. This, however, has the consequence that a substantial percentage of the sensors produced cannot be used, and this leads to a considerable increase in the price of the usable sensors.
U.S. Pat. No. 5,617,461 teaches that, making use of at least one calibration image, a defect image is produced. An image recorded from an object is then corrected making use of the defect image, this correction being carried out by means of linear interpolation.
U.S. Pat. No. 5465284 shows how a plurality of CCD sensors, which have been combined to form an image element, can be processed. The local resolution of the image is reduced in comparison with the resolution of the sensor in this way, whereas the signal-to-noise ratio is improved.
It is the object of the present invention to provide methods and devices for imaging in digital dental radioscopy, which permit the use of sensor arrays that are less expensive to produce while maintaining or improving the quality of the image produced.
According to a first aspect of the present invention, this object is achieved by a method for imaging in digital dental radioscopy making use of a sensor array, the individual image elements of which are smaller than a desired local resolution, so that a plurality of image elements forms a respective effective image element, comprising the steps of:
a) detecting first reference signals which are generated by the image elements of the sensor array when said sensor array is not exposed to X-radiation;
b) detecting second reference signals which are generated by the image elements of the sensor array when said sensor array is exposed to X-radiation;
c) determining defective image elements on the basis of the detected first and second reference signals; and
d) detecting third electric signals which are generated by the image elements of the sensor array when said sensor array is exposed to an X-radiation representative of an image of an object; and
e) producing the image by forming a respective output signal for the respective effective image elements using exclusively those third electric signals which are generated by image elements of the effective image element that have been determined as being non-defective.
Thus, according to the first aspect, the present invention provides a method for imaging in digital dental radioscopy making use of a sensor array in which a plurality of image elements forms a respective effective image element in the case of which defective image elements of the sensor array are initially determined on the basis of the detection of first and second reference signals. During the subsequent detection of the image of an object, only the image elements that have been determined as being non-defective are used. As has been stated hereinbefore, each effective image element comprises a plurality of image elements, the output signal for an effective image element being produced e.g. by taking an average of the detected signals of each image element of this effective image element. If defective image elements exist among the image elements, the output signal for the effective image element will, according to the present invention, be produced using exclusively those image elements of this effective image element that have been determined as being non-defective. It follows that the present invention permits sensor arrays including defective image elements to be used for dental radioscopy without any substantial deterioration of the quality of the image produced.
According to preferred embodiments of the present invention, defective image elements are determined by comparing the respective reference signals, which are detected for a specific image element, with a threshold value, an image element being determined as being defective when the reference signals of this image element deviate substantially from the threshold value. The value which may be used as a threshold value is e.g. the average of the first reference signals of all image elements or the average of the second reference signals of all image elements.
According to a second aspect of the invention, this object is achieved by a method for imaging in digital dental radioscopy making use of a sensor array having applied thereto a scintillation layer, the individual image elements of said sensor array being smaller than a desired local resolution so that a plurality of image elements forms a respective effective image element, comprising the steps of:
a) detecting electric signals which are generated by the image elements of the sensor array when said sensor array is exposed to an X-radiation that is representative of an image of an object;
b) assessing whether a signal detected for a respective image element indicates that the image element has been hit directly by an X-ray quantum, on the basis of signals which have been detected at least for neighbouring image elements; and
c) producing an image of the object by forming a respective output signal for the respective effective image elements on the basis of only those electric signals which are associated with the image elements of the respective effective image element and which have been assessed as having not been hit directly by an X-ray quantum.
Thus, according to the second aspect, the present invention provides a method for imaging in digital dental radioscopy making use of a sensor array of the above-mentioned type in the case of which electric signals representing the image of an object are detected by means of the sensor array. Subsequently, a signal detected for a respective image element will be assessed so as to find out whether the image element has been hit directly by an X-ray quantum. This assessment is carried out on the basis of at least the image elements bordering on the image element to be assessed. The signal detected for the image element to be assessed is preferably compared with an average obtained from a plurality of signals which have been detected for neighbouring image elements; if the signal detected for the image element to be assessed deviates from the above-mentioned average to a predetermined degree, it will be assessed that the signal indicates that the associated image element has been hit directly by an X-ray quantum. During the subsequent production of the image, exclusively the output signals which do not indicate any direct hit by an X-ray quantum will be used. It follows that the present invention permits, according to the second aspect, an elimination or at least an essential reduction of the image noise caused by direct hits, whereby the image quality will be improved substantially.
Furthermore, the two above-mentioned aspects may be combined in one imaging method so that the combined method permits, on the one hand, low-quality sensors, i.e. sensors with a large number of image element defects and/or column and line defects, to be used for digital dental radioscopy and improves, on the other hand, the image quality by eliminating or substantially reducing the image noise caused by direct hits.
According to a third aspect of the present invention, this object is achieved by a device for digital dental radioscopy comprising:
an X-radiation source;
a sensor array having applied thereto a scintillation layer; and
means for determining defective image elements on the basis of first reference signals, which are detected by means of the sensor array when said sensor array is not exposed to X-radiation, and on the basis of second reference signals which are detected when said sensor array is exposed to X-radiation,
wherein the individual image elements of the sensor array are smaller than a desired local resolution so that a plurality of image elements forms a respective effective image element; and
wherein means are provided for producing an image by generating an output signal for a respective effective image element using exclusively object signals which originate from image elements that have been determined as being non-defective.
According to a fourth aspect of the invention, this object is achieved by a device for digital dental radioscopy comprising:
an X-radiation source;
a sensor array having applied thereto a scintillation layer,
wherein the individual image elements of the sensor array are smaller than a desired local resolution so that a plurality of image elements forms a respective effective image element,
wherein means are provided for assessing whether an object signal of a respective image element indicates whether the image element has been hit directly by an X-ray quantum, on the basis of signals detected for at least the neighbouring image elements; and
wherein means are provided for producing an image using exclusively those object signals which do not indicate that the associated image element has been hit directly by an X-ray quantum.
Thus, the present invention additionally provides devices which are suitable for executing the above-described methods. In comparison with conventional devices for digital dental radioscopy, the devices according to the present invention can be produced at a moderate price, since, as has been explained above, also low-quality sensors can be used in these devices. Furthermore, the devices according to the present invention also permit the production of images having a higher quality by reducing the image noise caused by direct hits.
The present invention is based on the finding that sensor arrays whose image elements are smaller than the desired local resolution can be used for dental radioscopy even if individual image elements of the sensor array are defective or if individual image elements of the sensor array provide incorrect output signals. Such incorrect output signals of individual image elements may be caused e.g. by the above-described xe2x80x9cdirect hitsxe2x80x9d.
It follows that the present invention permits an improvement of the diagnostic expressiveness of dental X-radiographs while reducing simultaneously the production costs of the imaging devices by a reduction of the sensor costs.
Further developments of the present invention are specified in the dependent claims.